Determining the levels of neurotransmitters present in the living brain in real time is a matter of current scientific interest for research and clinicl reasons. Among these reasons is the need for understanding and mapping brain function and for improvement in the clinical application of deep brain stimulation (DBS). One analytical technique that holds potential promise in this application is fast-scan cyclic voltammetry (FSCV), however technical limitations have hindered its adoption for chronic in vivo use. Of particular difficulty has been the construction of a chronically-implantable FSCV electrode that possesses both the proper chemical properties for the monitoring of neurotransmitter levels as well as sufficient durability for chronic implantation in humans or animals. Carbon fiber has been used successfully under some circumstances, particularly at low voltage potentials, but at the higher voltages required for detection of neurotransmitters such as adenosine (e.g., up to +1.5V), its lifetime is very limited. Additionally, current FSCV methodology provides only a relative measure of neurotransmitter concentration or concentration change, but once an electrode is implanted chronically, recalibrating it on the bench becomes impossible. Due to buffer effects, proper bench calibration of a FSCV electrode intended for implantation is impossible in any case. Therefore, a means to extract absolute concentration data via FSCV with only in situ calibration would prove extremely valuable. Our work in these areas has been ongoing for several years, and initial results indicate that a coating of polycrystalline diamond film, when properly doped to provide electrical conductivity, yields electrodes that are both sufficiently sensitive and durable for chronic in vivo use. To construct these diamond-coated FSCV electrodes, our lab has already completed the construction of a chemical vapor deposition reactor to create polycrystalline diamond film-based electrodes. Initial results have been promising, but we propose to continue this development work. We have also determined that simple modifications to the FSCV procedure, combined with more sophisticated data analysis procedures, appear to allow for the determination of absolute analyte concentration. These two lines of work are mutually-reinforcing insofar as they are both focused on the goal of a long-term implantable FSCV neurotransmitter-monitoring device and, ultimately, a closed-loop DBS stimulator.